The four Jovian or Galilean Moons—Io, Europa, Ganymedes & Callisto.
Photographs made by the Galileo spacecraft between 1996-97.
First seen with a home-made 8x telescope in 1610.

Upon their discovery, it became clear that the old Ptolemaic view that
the Universe revolved around the Earth was incorrect,
and that the Earth actually was, like Jupiter, a planet
that circled around the Sun.